Remove letters-related code (#175)

This deletes a big ol' chunk of code related to letters. It's not everything—there are still a few things that might be tied to sms/email—but it's the the heart of letters function. SMS and email function should be untouched by this.

Areas affected:

- Things obviously about letters
- PDF tasks, used for precompiling letters
- Virus scanning, used for those PDFs
- FTP, used to send letters to the printer
- Postage stuff

app/v2/errors.py

@@ -49,11 +49,6 @@ class ValidationError(InvalidRequest):
         self.message = message if message else self.message
 
 
-class PDFNotReadyError(BadRequestError):
-    def __init__(self):
-        super().__init__(message='PDF not available yet, try again later', status_code=400)
-
-
 def register_errors(blueprint):
     @blueprint.errorhandler(InvalidEmailError)
     def invalid_format(error):